The present invention generally concerns, and essentially has as its purpose, a process for purification of a gas, in particular a natural gas containing C1 to C5 hydrocarbons and heavier, in order to eliminate sulphur compounds of the volatile mercaptan type without gas adsorption or cryogenic expansion processes, for example in a turbo-expander. It also concerns the products obtained by implementation of this process and an installation for application of the process.
It is known that a gaseous fluid, effluent of a natural gas reservoir, generally contains substances of nauseous odour formed mainly from mercaptans such as, for example:
methyl mercaptan or methanethiol, chemical formula CH4S, which is highly volatile and boils at 5.9xc2x0 C.,
ethyl mercaptan or ethanethiol, chemical formula C2H6S which is less volatile and boils at 35xc2x0 C.,
propanethiols including, in particular, isopropyl mercaptan and n-propyl mercaptan, and
increasingly heavy mercaptans which are therefore less and less volatile.
The presence of mercaptans in natural gas extracted from a reservoir presents the following disadvantages for a natural gas: firstly a nauseous odour, secondly high toxicity, and finally, the presence of impurities likely to be decomposed into hydrogen sulphide and disulphide and therefore corrosive, especially for the pipes transporting such a fluid.
A known technical problem is how to obtain, from the effluent of a natural gas reservoir, on the one hand natural gas in which the level of mercaptans has been reduced to 10 parts per million so that it can be used in accordance with commercial specifications for its transportation by gas pipeline to the customer or distributor and, on the other hand, recovery products made up, in particular, of stabilized gasoline consisting of C5 hydrocarbons and heavier.
As the prior technique stands, this known technical problem can be resolved in particular by one of the following three processes:
1) The molecular sieve adsorption process at an effective pressure of about 60 bar.
This process presents the disadvantage of producing a gas which results in the regeneration of the adsorbent and which is rich in mercaptans (i.e. containing about 20 times the level of mercaptans of the gas to be processed) whose discharge is prohibited by local environmental protection regulations and which represents either gas losses or subsequent processing costs which are prohibitively high.
2) The high pressure absorption process (for example 60 bar) in a heavy hydrocarbon, for example C7 or heavier.
This process presents the disadvantage of producing intermediate products, for example propane-based, which cannot always be made use of and which must therefore be reinjected into the purified gas with circulation of huge quantities of C7 hydrocarbon solvents and heavier, resulting in an increase in the cost of the absorber and the use of large quantities of heat in the reboilers and refrigeration in the coolers
3) The cryogenic processing procedure in which one common mode of operation presents the disadvantage of expanding the source gas which then has to be recompressed in order to deliver it to the distribution gas pipeline. This process also results in the production of intermediate products, for example propane-based which cannot always be made use of and which must therefore be reinjected into the purified gas, resulting in the need to use several compressors which are costly in terms of equipment and energy, as well as the need for intensive drying of the source gas to reduce its moisture content to less than 0.1 part per million.
Such a process of cryogenic processing is described for example in :
U.S. Pat. No. 3,763,658, published on Oct. 9, 1973, which presents the disadvantage of high running costs, particularly in terms of high operating pressure;
U.S. Pat. No. 5,655,109, published on Aug. 19, 1997, whose running costs are markedly lower in terms of high operating pressures than those of the previous patent, and which therefore represents an appreciable improvement over the previous patent.
The main purpose of the present invention is to eliminate the disadvantages of the aforementioned known processes whilst retaining their advantages by creating a process which avoids the production of propane, for example, when this product is not desired, which avoids the expansion of the source gas and which avoids the use of large quantities of absorption oil, particularly C7 hydrocarbon-based or heavier. The invention therefore represents a novel solution to a technical problem because of its specific definition as described above. The invention thus offers the important advantage of much lower running costs than those of the aforementioned known processes and therefore represents a perfecting or appreciable improvement in relation to the known processes.
To this end, the process according to the invention consists in successively passing the previously dried and partially cooled source gas through an absorption column under pressure which receives a reflux fluid and whose head product is a purified gas, with a mercaptan content of less than 30 parts per million, and whose base product supplies a fractionating column which separates the most volatile compounds of the source gas at the top, free from mercaptans, from the gasoline fraction of said source gas withdrawn from the bottom and containing mercaptans, the most volatile of which are eliminated at the top of at least one distillation column whose base product, consisting of liquid gasoline with a mixture of C5 hydrocarbons and heavier, processed by sweetening such that it no longer contains volatile mercaptans, is partially mixed with the most volatile compounds separated at the top of the natural gas fractionating column and then recompressed, this mixture being cooled down and sent by reflux to the absorption column to constitute said reflux fluid in it.
In the sense of the present description, the most volatile compounds of the source gas will be considered free from mercaptans once their mercaptan level is less than 30 parts per million.
In addition, the liquid gasoline processed by sweetening will be considered to be no longer containing any volatile mercaptans once its volatile mercaptan level is less than 5 ppm, and preferably less than 1 ppm.
According to another characteristic of the invention, the main stream of source gas is subdivided beforehand into two partial bypass streams, one of which, previously cooled, supplies the lower section of the aforementioned absorption column, and the other, previously cooled, supplies an intermediate zone of said column.
According to another characteristic of the invention, the base product of the aforementioned absorption column, containing C2 hydrocarbons and heavier and mercaptans, is previously expanded before supplying a fractionating column enabling the separation of the most volatile compounds of the source gas from the mercaptans.
According to yet another characteristic of the invention, the base product of the aforementioned fractionating column, containing C3 hydrocarbons and heavier and mercaptans, supplies, after expansion, a first gasoline distillation column, preferably functioning as a depropanizer, and whose base product contains C4 hydrocarbons and heavier, and mercaptans.
In accordance with yet another characteristic of the invention, the head product of the aforementioned fractionating column, free from mercaptans and containing C1, C2 and C3 hydrocarbons, is cooled by partial condensation then separated, on the one hand, into a liquid phase which is sent to said column and, on the other hand, a gaseous phase which is recompressed before being mixed, as mentioned above, with at least a part of the distillation end product of the aforementioned gasoline fraction whose other part is extracted.
The head product of the fractionating column will be considered to be free from mercaptans once its mercaptan level is less than 30 ppm.
According to yet another characteristic of the invention, the base product of the aforementioned distillation column, containing C4 hydrocarbons and heavier, and mercaptans, supplies, after expansion, a second gasoline distillation column, preferably functioning a debutanizer, whose base product contains the residual gasoline fraction consisting of C5 hydrocarbons and heavier, and non-volatile mercaptans.
According to yet another characteristic of the invention, the head product of said first distillation column, containing C3 hydrocarbons and methylmercaptan, is cooled by undergoing complete condensation and then separated into two partial liquid streams, one of which is sent to the head of said column and the other of which undergoes a sweetening process to eliminate methylmercaptan, especially methylmercaptan in mixture with a sweetening agent, and to obtain a mixture of C3 hydrocarbons free from mercaptans, that is to say having a mercaptan level less than 5 ppm, and preferably less than 1 ppm.
According to another characteristic of the invention, the head product of the second distillation column, containing C4 hydrocarbons and residual volatile mercaptans, is cooled by complete condensation and then separated into two partial liquid streams, one of which is sent to the head of said column and the other of which undergoes a sweetening proecss to eliminate residual volatile mercaptans, especially those in mixture with a sweetening agent, and to obtain a mixture of C4 hydrocarbons free from mercaptans, that is to say having a mercaptan level less than 5 ppm, and preferably less than 1 ppm.
According to another characteristic of the invention, the partial stream of source gas to be cooled and the aforementioned reflux gas are cooled by the gas emitted at the head of said absorption column and, possibly, by an auxiliary cooling fluid, such as propane for example, the gas emitted from the column being, in this way, reheated by thermal exchange until it is close to room temperature.
The invention also concerns other products obtained by implementation of the aforementioned process, such products consisting, in particular, of processed gas free from mercaptans and, more precisely, containing less than 30 ppm of mercaptans, possibly consisting of a mixture of C3 hydrocarbons and C4 hydrocarbons and a gasoline comprised of a mixture of C5 hydrocarbons and heavier.
Finally, the invention concerns an installation for application of the aforementioned process and comprised of an absorption column under pressure to which are successively connected, from bottom to top, supply pipes for the source and reflux gas, the main supply pipe for the source gas branching into two bypass lines, one of which, along with the reflux pipe, passes through an heat exchanger in the same direction, and whose column head is connected to an outlet pipe for the processed gas passing through said heat exchanger, preferably in the opposite direction, while its base is connected by means of a pipe to a fractionating column, the base of which is connected by means of a pipe to at least one gasoline distillation column whose base is connected by means of an outlet pipe to the delivery pipe of a compressor having a suction section connected, by means of another pipe, to the head of the fractionating column, the outlet pipe and the delivery pipe joining at said reflux pipe.
According to another characteristic of the invention, the head of the fractionating column is connected by means of a pipe passing through a cooler-condenser to the inlet of a phase separator having a gas phase outlet connected by means of a pipe to the suction section of the aforementioned compressor whilst its liquid phase outlet is linked by means of a pipe to the upper section of said column.
According to yet another characteristic of the invention, the gasoline distillation system is comprised of two columns in series whose respective heads are each connected, by means of their respective pipes passing through a cooler-condenser, to the inlet of a condensate reservoir whose outlet opens into, on the one hand, a return pipe at the upper section of the associated column and, on the other hand, a header pipe towards a processing or utilization apparatus.
In accordance with another characteristic of the invention, the aforementioned header pipe, linked to the first distillation column, opens into the inlet to a sweetening apparatus whose outlet is connected to a C3 hydrocarbon reservoir whilst the aforementioned header pipe, linked to the second distillation column, opens into a C4 hydrocarbon sweetening or utilization system.
According to yet another characteristic of the invention, a bypass line for C5 hydrocarbons and heavier is connected to the outlet pipe at the base of the second distillation column.
In this way, the results obtained by the invention lead to significant advantages in terms of simplifying and increasing the savings made in the construction and technology required for the equipment used and the methods for their implementation.
The invention will be better understood and other characteristics, details and advantages of the invention will become clearer in the course of the description which follows below, with reference to the diagrammatic drawings in the appendices. These drawings are given purely for the purpose of example and are in no way restrictive.